The invention relates to NOx catalyst systems. More specifically, this invention relates to a method for controlling parasitic hydrocarbon oxidation in high temperature NOx catalyst systems.
Internal combustion engines produce large amounts of exhaust gases consisting primarily of carbon dioxide (CO2), water, unburned hydrocarbons (HCs), carbon monoxide (CO) and oxides of nitrogen (NOx). Since the 1970""s the emission of unburned HCs, CO and NOx has been regulated and the world-wide regulatory climate for reducing exhaust gases has become ever more stringent. As a result, many present day engines, especially gasoline-fueled engines used for passenger automobiles and the like, operate very near stoichiometric conditions, where catalyst technology that allows simultaneous abatement of unburned HCs, CO, and NOx, is well advanced. There is a desire to introduce diesels and gasoline lean-burn vehicles on a broader basis because of their significant fuel economy advantages with attendant lower fuel costs. These vehicles operate with a ratio of air to fuel in the combustion mixture supplied to the engine that is maintained above the stoichiometric ratio so that the resulting exhaust gases are xe2x80x9clean,xe2x80x9d i.e., the exhaust gases are relatively high in oxygen content and relatively low in reductants content, e.g., HC, CO, and/or hydrogen (H).
Although lean burn engines provide enhanced fuel economy, they have the disadvantage that conventional three-way catalysts (TWC) cannot adequately abate the NOx component of the pollutants in the gas stream. Considerable research is currently underway toward the development of catalysts that are capable of decomposing or reducing NOx under oxidizing conditions. Much of this research focuses on using hydrocarbons in engine exhaust to catalytically reduce oxides of nitrogen under lean conditions. While advances are being made, there is a need in the industry for increased efficiencies of lean NOx catalyst systems.
The invention disclosed herein addresses these and other concerns.
In accordance with the present invention, the short-comings and disadvantages of the prior art are overcome by utilizing a method of NOx abatement comprising: generating a gas stream containing hydrocarbons and NOx, maintaining the temperature of the gas stream within the range of about 150xc2x0 C. to about 550xc2x0 C., controlling the residence time of said hydrocarbons upstream of a NOx abatement catalyst so as to reduce the rate of hydrocarbon oxidation, and contacting the gas stream with said NOx abatement catalyst.